In an electrical discharge machine, when water is used as a dielectric fluid, electrical insulation is required. For instance, ion-exchanged water is produced by ion-exchange resin formed from water-purifying resin into which are mixed an H+-type(a hydrogen ion) positive ion exchange resin and an OH−-type(a hydride ion) ion exchange resin. Machining is performed by use of the ion-exchanged water.
Cations contained in service water, such as sodium ions (Na+) and calcium ions (Ca2+), contact the H+-type positive ion exchange resin, whereupon the positive ions are exchanged with H+ ions. Further, as a result of negative ions, such as chloride ions (Cl−) and sulfate ions (SO42−), contacting the OH−-type negative ion exchange resin, the negative ions are exchanged with the OH− ions. Consequently, impurities contained in the service water employed as a dielectric fluid are removed, and H+ ions and OH− ions bond together to thus generate water, whereby a dielectric fluid required for electric discharge machining can be produced.
The conductivity of an impurity-removed dielectric fluid suitable for electric discharge machining is generally 70 μS/cm or less. A pH value is determined by the degree of the concentration of hydrogen ions. However, the OH− ions exchanged with hydrogen carbonate ions (HCO3−) diffuse into the dielectric fluid, and hence extreme changes do not appear in the degree of the concentration of the hydrogen ions. The dielectric fluid usually exhibits a pH value of 7.
In an electric discharge machine using the water-based dielectric fluid, a pulse voltage is applied between a workpiece and an electrode while the dielectric fluid is being squirted to the workpiece or while the workpiece is being immersed in the dielectric fluid, to thus effect electric discharge and machining. As a result of the workpiece being immersed in the dielectric fluid for a long period of time, metal forming the workpiece is known to be corroded away, so that the quality of the workpiece is deteriorated.
For this reason, there has been proposed a method for preventing corrosion of a workpiece by sandwiching an insulation material between a table and the workpiece; taking the table as an anode and the workpiece as a cathode; and applying a voltage to the workpiece during non-machining operation (e.g. Patent Document 1).
Moreover, there has also been proposed a method for preventing corrosion of a workpiece by taking the table or the workpiece set in a working tank as a cathode, and taking as an anode a second electrode provided on the table or the workpiece through the dielectric fluid; and applying a minute electric current and a minute voltage to the cathode and the anode (e.g. Patent Document 2).
Also, there has been proposed a technique to prevent corrosion of metal by causing water to flow through ion exchange resin to thus control pH of water (e.g. Patent Document 3).
Patent Document 1: JP-A-2004-291206
Patent Document 2: JP-A-5-220618
Patent Document 3: JP-A-1-164489